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Química Nova

Print version ISSN 0100-4042On-line version ISSN 1678-7064

Quím. Nova vol.30 no.5 São Paulo Sept./Oct. 2007

https://doi.org/10.1590/S0100-40422007000500009 

ARTIGO

 

Clerodane diterpenes from leaves of Casearia sylvestris Swartz

 

 

André G. dos Santos; Carla C. Perez; Aristeu G. Tininis; Vanderlan da S. Bolzani and Alberto J. Cavalheiro*

Instituto de Química de Araraquara, Universidade Estadual Paulista "Júlio de Mesquita Filho", R. Francisco Degni, s/n, 14800-900 Araraquara - SP, Brasil

 

 


ABSTRACT

Ethanolic extracts of the leaves of Casearia sylvestris yielded a novel clerodane diterpene, 15-hydroxy-3-cleroden-2-one, together with the known diterpenes (-)-hardwickiic acid, reported for the first time from this species, and casearins B and G, previously isolated from C. sylvestris. The structures of all four compounds were determined by spectrometric analysis. The new clerodane diterpene and (-)-hardwickiic acid contain structural features that are completely different from the highly oxygenated casearins and casearvestrins isolated from C. sylvestris.

Keywords: Casearia sylvestris; clerodane diterpenes; 15-hydroxy-3-cleroden-2-one.


 

 

INTRODUCTION

Casearia sylvestris Swartz (Flacourtiaceae) is a tree that is widely distributed within various ecosystems of South America, such as the Cerrado and the Atlantic and Amazon forests1. In the popular medicine of Brazil, the use of the plant1 is correlated with its pharmacological properties including anti-inflammatory2, anti-ophidian2,3 and anti-ulcer1,4 activities. A number of phytochemical investigations of species of Casearia5-9 have revealed the occurrence within the genus of oxygenated tricyclic clerodane diterpenes exhibiting a cis configuration between rings A and B and a characteristic diacetal system in ring C at positions C-18 and C-19. Several compounds of this type have been isolated from C. sylvestris, specifically casearins and casearvestrins, and they exhibited cytotoxic activity5,7-9.

This paper describes the isolation and structure elucidation of a novel clerodane diterpene, 15-hydroxy-3-cleroden-2-one (1), from the leaves of C. sylvestris, together with (-)-hardiwickiic acid (2), reported for the first time from this species. In addition, casearin B (3) and G (4) were isolated, diterpenes that have been previously detected in the species5,7,8. The structures of compounds 1-4 (Figure 1) were deduced on the basis of their spectral data and comparison with appropriate values reported in the literature.

 

 

RESULTS AND DISCUSSION

Ethanol extract of the leaves of C. sylvestris was submitted to chromatographic fractionation and yielded compounds 1-4. In the positive-ion mode, the HRTOF-ESIMS of 1 exhibited an [M+Na]+ ion at m/z 329.2481 that was compatible with a molecular formula of C20H34O2. Analysis of the PND and DEPT 135º 13C-NMR spectra of 1 revealed twenty signals similar to those of the clerodane diterpene skeleton6-11, being five of them attributed to methyl groups: C-16 (d 19.7), C-17 (d 16.0), C-18 (d 20.5), C-19 (d 32.2), C-20 (d 19.3). The IR and UV absorptions at 1653 cm-1 and at 218 nm (lmax) and the NMR signals at d 199.3 (C-2), 128.6 (C-3), 168.5 (C-4) and 5.84 br s (H-3) indicated the presence of an a,b-unsaturated ketone group. In addition, the IR absorption at 3448 cm-1 and the NMR signals observed at d 61.2 (C-15) and 3.68 m (H-15) are characteristic of a hydroxymethine group. The 1H NMR and 13C assignments (Table 1) together with the results of HMQC, COSY and HMBC (Figure 2) experiments led to structure of compound 1. The relative configuration assignment at C-5, C-8, C-9 and C-10 was firstly proposed based on comparison of chemical shifts of methyl groups C-17, C-19 and C-20 of compound 1 with literature data, as follow. The cis stereochemistry at the junction of rings A and B in compound 1 could be deduced from the chemical shift of C-19 (d 32.2) when compared with C-19 chemical shifts of cis-clerodane diterpenes, for example floridiolic acid12 and 13-hydroxy-cis-ent-cleroda-3,14-diene 13 (d 33.6 and 33.0, respectively). In contrast, the trans-clerodane diterpenes 2-oxokolavenic acid10 and eremone11 presented chemical shifts for this methyl carbon at d 19.5 and 18.8, respectively. In clerodane diterpenes with a cis configuration between methyl groups C-17 and C-20, such as hautriwaic acid11, 2-oxokolavenic acid10 and floridiolic acid12, the chemical shift of C-20 was observed at approximately d 18.0, whilst clerodane diterpenes with a trans configuration between these methyl groups, as in the casearins and casearvestrins5-9, show chemical shift for C-20 at approximately d 26.0. So, the chemical shifts attributed to C-20 in compound 1 (19.3) is indicative of a cis relationship between these methyl groups. These propositions were confirmed by the NOE enhancements arising from the dipolar interactions between H-19 (1.22 s) and H-10 (1.87 br d; 6.5 Hz) and between H-17 (0.77 d; 7.0 Hz) and H-20 (0.57 s). In addition, correlations observed in the NOE experiments (Figure 3) between H-1a (d 2.71) and H-19 and between H-1b and H-20, revealed the trans relationship between C-17/C-20 and C-19 methyl groups. A diastereomer of compound 1 was previously isolated from Cistus populifolius L. (Cistaceae)14, but the 13C NMR data of this compound were not included in the article to a complete comparison.

 

 

 

 

 

 

In the positive-ion mode, the HRTOF-ESIMS of compound 2 exhibited an [M+H]+ ion at m/z 317.2117, compatible with a molecular formula of C20H28O3, which was confirmed by the finding of [M+Na]+ ion at m/z 339.1963. Analysis of the PND 13C-NMR spectra revealed twenty signals similar to those of the clerodane diterpene skeleton6-11 and showed the presence of three methyl groups: C-17 (d 16.0), C-19 (d 20.6) and C-20 (d 18.3). IR absorptions presented at 3397 cm-1 and 1684 cm-1, UV absorption (lmax) at 210 nm and the signals observed in the 13C-NMR spectra at d 172.5 (C-18), 140.2 (C-3) and 141.5 (C-4) were indicative of an a,b-unsaturated carboxylic group. In addition, the 1H and 13C NMR spectra showed signals that could be ascribed to a b-mono-substituted furan ring (C-13, C-14, C-15 and C-16) in the lateral chain of the diterpene. The complete attribution of the NMR signals (table 1) was based on the correlations observed in the HMQC, HMBC and COSY contour plots and this attribution is in agreement with the NMR data from literature for hardiwickiic acid15. The negative value of [a]D25 confirmed the absolute configuration as (-)-hardiwickiic acid16. Compound 2 or its enantiomer were previously isolated from other species of different families, as Copaifera duckei, C. guianensis, Croton californicus, C. aromaticus, Hardwickia pinnata, Salvia divinorum and Sindora sumatrana15,17, and now we found it for the first time in a Casearia species.

The 1H- and 13C-NMR data of 3 (C31H44O10) and 4 (C29H42O8) showed signals identical of the compounds previously reported in C. sylvestris5,7,8, casearins B and G, respectively.

 

EXPERIMENTAL

General

Column chromatography (CC) and solid phase extraction (SPE) were performed over activated charcoal (Synth), silica gel G (40-63 µm; Merck) or RP-C18 (40-63 µm; Merck) and the resulting fractions were monitored by TLC, HPLC-UV and 1H-NMR. Comparative TLC was carried out on silica gel G60 layers (0.25 mm thickness; Merck). HPLC-UV analyses were conducted using a Supelcosil LC-18 column (250 x 4.6 mm; i.d. 5 µm) connected to a Varian chromatographic system consisting of a ProStar 240 solvent delivery module, a ProStar 330 photodiode array detector, a ProStar 410 autosampler and a Star chromatography workstation. For preparative HPLC, a Supelcosil PLC-18 column (250 x 21.2 mm; i.d. 12 µm) was coupled to a Varian system consisting of Dynamax model Sd-1 solvent delivery system and a ProStar 320 detector with Star integrator software.

IR spectra (KBr disc) were obtained using a Perkin Elmer FTIR 1600 spectrometer, UV spectra were measured on a Cary 13E instrument and a Perkin Elmer 341 polarimeter was employed to determine optical activities. NMR spectra of 1 and 2 were recorded at 500 and 125 MHz for 1H- and 13C-NMR, respectively, using a Varian INOVA 500 spectrometer with CHCl3 as internal standard. The NMR spectra of 3 and 4 were measured at 200 and 50 MHz for 1H- and 13C-NMR, respectively, on a Bruker AC-200 instrument using the same internal standard. Accurate-mass measurements were performed on an ESI-quadrupole-time of flight instrument (UltrOTOFQ, Bruker Daltonics, Billerica, MA).

Plant material

Leaves of Casearia sylvestris Swartz (Flacourtiaceae) were collected at the Carlos Botelho State Park (São Paulo State, Brazil) and in Araraquara (São Paulo State, Brazil) between August and November 1999. Voucher specimens (AGS13 and AGS45) are deposited at the State Herbarium "Maria Eneida P. Kaufmann" of the Botanic Institute (São Paulo State, Brazil).

Extraction and isolation of 1, 3 and 4

Dried and powdered leaves of C. sylvestris (2.1 kg) were extracted by sonication with EtOH (3 x 6.0 L; 20 min per extraction) and concentrated under reduced pressure to yield 139.0 g of residue. The crude extract was partitioned between MeOH:water (7:3, v/v, 0.8 L) and CH2Cl2 (1.5 L) and the CH2Cl2 layer separated, dried and concentrated under reduced pressure to yield 71.0 g of a residue that was subjected to SPE over silica gel eluted sequentially with CH2Cl2, CH2Cl2:MeOH (9:1) and MeOH, yielding 8 fractions. SPE-fraction 5 (13.5 g) eluted with CH2Cl2:MeOH (9:1) was submitted to CC over silica gel eluted sequentially with hexane:EtOAc (8:2, 6:4 and 4:6), EtOAc, EtOAc:MeOH (9:1) and MeOH, yielding 12 CC-fractions. CC-fraction 8 (1.45 g) eluted with hexane:EtOAc 4:6 was submitted to RP-CC over octadecyl silane eluted sequentially with MeOH:water (6:4, 8:2 and 9.5:0.5), MeOH and CH2Cl2. Sub-fraction 3 (120.0 mg) eluted with MeOH:water 8:2 was submitted to preparative RP-HPLC (C-18, 65% MeOH, isocratic mode, flow rate 13.0 mL/min, l 235 nm) to yield 1 (5.0 mg). Further, CC-fraction 7 (2.4 g) eluted with hexane:EtOAc 6:4 was submitted to preparative RP-HPLC (C-18, 75% MeOH, isocratic mode, flow rate 10.0 mL/min, l 235 nm), yielding compounds 3 (86.2 mg) and 4 (30.9 mg).

Extraction and isolation of 2

Dried and powdered leaves of C. sylvestris (2.0 kg) were extracted by maceration under agitation with EtOH (3 x 3.0 L; 2 h per extraction) and concentrated under reduced pressure to yield 190.0 g of residue. The crude extract was partitioned between EtOH/Water (6:4, v/v, 1.5 L) and hexane (1.5 L), and the hexane layer separated, dried and concentrated under reduced pressure to yield 120.2 g of a residue that was subjected to SPE over silica gel:activated charcoal (1:1) eluted sequentially with hexane:EtOAc (7:3 and 3:7) and MeOH. The fraction eluted with hexane/EtOAc 3:7 (28.4 g) was submitted to CC over silica gel eluted sequentially with hexane:EtOAc (7:3, 6:4, 1:1, 4:6 and 3:7) and EtOAc, yielding 24 fractions. Fraction 11 (34.0 mg) eluted with hexane:EtOAc 1:1 was submitted to SPE over RP-C18 eluted with MeOH:water (98:2) to yield 2 (29.0 mg).

15-hydroxy-3-cleroden-2-one (1)

White powder (5.0 mg). [a]D20 +42º (c 0.14; MeOH). UV lmax nm (MeOH) 218 (e = 10.5 x 103 cm3 mol-1 cm-1). IR nmax cm-1 3448, 2958-2853, 1653, 1037 (KBr). 1H- and 13C-NMR see Table 1. HRTOF-ESIMS m/z 329.2481 [M+Na]+ (calcd for C20H34O2Na+ , 329.2451).

(-)-Hardiwickiic acid (2)

White powder (29.0 mg). [a]D25 -35º (c 1.0; MeOH). UV lmax nm (MeOH) 210 (e = 7.2 x 103 cm3 mol-1 cm-1). IR nmax cm-1 3397, 2958-2864, 1684, 1640, 1257, 1078 (KBr). 1H- and 13C-NMR see Table 1. HRTOF-ESIMS m/z 317.2117 [M+H]+ (calcd for C20H29O3, 317.2117) and m/z 339.1963 [M+Na]+ (calcd for C20H28O3Na+ , 339.1930).

 

CONCLUSIONS

The new compound (+)-15-hydroxy-3-cleroden-2-one (1) and (-)-hardwickiic acid (2) are the first diterpenes reported in C. sylvestris without the typical highly oxygenated backbone. Casearins and casearvestrins, previously isolated from this species, have common stereochemical features of a cis-clerodane diterpene, notably a cis configuration at the junction of rings A and B and, in addition, trans configuration between the methyl groups at C-17 and C-205,7-9. However, whilst the newly described (+)-15-hydroxy-3-cleroden-2-one (1) is a typical cis-cledorane, (-)-hardiwickiic acid (2) possesses a trans-clerodane configuration. In conclusion, the occurrence of compounds 1, 2, 3 and 4 in C. sylvestris reveals the ability of this species in biosynthesize both cis and trans-clerodanes as it is common in other plant species as Adelanthus lindenbergianus (Adelhantaceae)13 and Grangea maderaspatana (Asteraceae)18.

 

ACKNOWLEDGMENTS

The authors are grateful to the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and the Instituto Florestal da Secretaria do Meio Ambiente do Estado de São Paulo for support of the project; Laboratório de Espectrometria de Massas da Faculdade de Ciências Farmacêuticas da Universidade de São Paulo, campus de Ribeirão Preto for MS determination.

 

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Recebido em 18/4/06; aceito em 10/11/06; publicado na web em 17/7/07

 

 

* e-mail: albjcava@iq.unesp.br

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